This invention relates to control of solid-state image sensors used, for example, in a video camera or the like.
Known expedients for implementing an exposure correction function in a video camera or the like include a method using a diaphragm or the like for directly controlling the amount of light incident upon an image sensor, and a method using an electronic-shutter function for controlling the charge storage time of an image sensor.
However, of the conventional methods described above, the former method, using diaphragm blades, requires not only the diaphragm blades themselves but also mechanical members peripheral thereto, as a result of which a video camera using this method is unduly large and costly. The latter method using the electronic-shutter function is disadvantageous in that the motion of the sensed image is unnatural.
The problem relating to the method using the electronic-shutter function will be described in greater detail. FIG. 1 is a plan view schematically illustrating the structure of an interline transfer-type CCD. Numeral 1 denotes a sensor which performs a photoelectric conversion, 2 a vertical transfer register, 4 a horizontal transfer register, and 5 an output amplifier.
FIG. 2 is a sectional view taken along line A--A' of the CCD of FIG. 1 and a diagram illustrating the states of potentials at various portions corresponding to this cross section.
In FIG. 2, numeral 6 denotes a channel stop (CS) for pixel separation, 7 a read-out gate (ROG) for transferring the electric charge, which has accumulated in the sensor portion 1, to the vertical transfer register 2, 8 a substrate, and 9 an oxide film.
The operation of the conventional electronic shutter will now be described with reference to FIGS. 2 and 3.
FIG. 3 is a timing chart of various signals in one field interval T of a standard television signal. A .phi.ROG pulse is a pulse applied to the read-out gate (ROG) 7. When the logic level of this pulse is "H" (high), the potential of the read-out gate (ROG) 7 falls and the electric charge in the sensor portion 1 is transferred to the vertical transfer register 2. An elimination pulse .phi.SUB is applied to the substrate 8. When this pulse is "H", the electric charge which has accumulated in the sensor portion 1 is swept out (eliminated) to the exterior through a .phi.SUB terminal.
In this example, as shown in FIG. 3, the .phi.ROG pulse is applied to the CCD during the vertical retrace interval, and the .phi.SUB pulse is applied to the CCD during the horizontal retrace interval. After the .phi.ROG pulses is applied and the electric charge of the sensor portion 1 is transferred to the vertical transfer register 2 at time t.sub.0, the next field interval begins. However, since .phi.SUB attains the "H" level at every horizontal retrace interval from time t.sub.0 to time t.sub.1, the electric charge which has accumulated from t.sub.0 to t.sub.1 does not remain in the sensor portion 1. Since .phi.SUB is at the "L" level from time t.sub.1 to time t.sub.2, electric charge is stored in the sensor portion 1 during this time period. This charge is transferred to the vertical transfer register 2 by the "H"-level .phi.ROG pulse applied at time t.sub.2. The end result is that the exposure time of the CCD in this case becomes (t.sub.2 -t.sub.1).
Accordingly, though the conventional electronic shutter adequately performs the function of an electronic shutter, problems are encountered when it is applied to correction of exposure. In particular, when exposure time is varied in a continuous manner, a difference in dynamic resolution from one exposure time to another is evident on the picture. As a consequence, the picture that results is highly unattractive since the dynamic resolution changes from one field to the next.